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Income Taxes - Schedule of Significant Components of Deferred Tax Assets and Liabilities (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Deferred tax assets:    
Research and development tax credit carryforwards $ 20,019 $ 13,967
Net operating loss carryforwards 4,918 4,902
Stock-based compensation 2,793 4,066
Investment tax credit carryforwards 2,181 1,576
Inventory reserves 2,059 3,143
Vacation accrual 1,255 1,928
Contingent consideration liabilities 148  
Unrealized loss on investments 135 136
Deferred revenue 79 154
Warranty reserves 45 73
Bad debt reserves 36 52
Alternative minimum tax credit carryforward   340
Other 273 331
Total deferred tax assets 33,941 30,668
Less: Valuation allowance for deferred tax assets (33,024) (29,274)
Net deferred tax assets 917 1,394
Deferred tax liabilities:    
Prepaid expenses (470) (654)
Patent amortization (161) (296)
Depreciation (76) (406)
Total deferred tax liabilities (707) (1,356)
Net deferred tax assets (liabilities) $ 210 $ 38